The present invention relates to a surge arrester for use in a substation and, more particularly, to a tank-type surge arrester connected to a gas-insulated switchgear or a power transformer.
Standards specified for gas-insulated tank-type surge arresters in Surge Arrester Standards JBC-2372-1995 are applied to high-performance surge arresters of voltage capacities in the range of 187 kV to 500 kV. Such a surge arrester employs zinc oxide elements having a reference voltage on the order of 200 V/mm as basic elements. Zinc oxide elements are connected simply in series to construct a surge arrester. Such a surge arrester is too long to use the same as a tank-type surge arrester for a gas-insulated switchgear. Therefore, the zinc oxide elements are arranged on four poles and are connected in a zigzag arrangement to form the surge arrester in a shorter length so that the surge arrester can be constructed in a reduced height.
A zinc oxide element having a reference voltage of, for example, 400 V/mm can be formed by increasing the resistance of the zinc oxide element by forming the same of zinc oxide grains of smaller grain sizes to increase the number of zinc oxide grains connected in series. Thus, the working voltage of the zinc oxide element can be increased and thereby the length of the zinc oxide element can be reduced by about half. If the length of the zinc oxide element can be thus reduced, a tank-type surge arrester for a gas-insulated switchgear can be constructed by stacking zinc oxide elements linearly instead of arranging the zinc oxide elements in three or four columns and connecting the same in a zigzag connection. For example, a tank-type surge arrester having a voltage capacity in the range of 154 to 500 kV and comprising zinc oxide elements linearly connected in series can be realized. Thus, a gas-insulated tank-type surge arrester can be miniaturized by using a reduced number of zinc oxide elements each having an increased resistance.
However, a high-voltage withstanding zinc oxide element having a high withstand voltage and a high resistance is formed of zinc oxide grains of small grain sizes and hence the capacitance of such a zinc oxide element is smaller than that of the conventional zinc oxide element. A tank-type surge arrester employing zinc oxide elements having a small capacitance is susceptible to the influence of stray capacitance capacity relative to a grounding tank and voltage is irregularly distributed to the elements. Consequently, in a high-performance surge arrester for use at a high voltage applied ratio, the voltage applied life characteristic of the elements is deteriorated due to irregular voltage distribution and the surge arrester become practically unusable.
A method of controlling voltage distribution to the conventional zinc oxide elements disclosed in Japanese Patent Laid-Open No. 55-105989 or European Patent Publication No. EP0634757B1 discretely arranges coaxial metal shield rings including a head shield ring and a tail shield ring of the same diameter from a high-voltage side.
A tank-type surge arrester shown in FIG. 10 employs the method mentioned in Japanese Patent Laid-Open No. 55-105989 or European Patent Publication No. EP0634757B1. In this tank-type surge arrester, voltage distribution indicated by a dotted line in FIG. 9 is about 1.18 and voltage applied ratio is on the order of 100%. Therefore, this tank-type surge arrester is not suitable for use at a high voltage applied ratio. Usually, voltage applied ratio for a tank-type surge arrester is set at 90% or below to ensure that the zinc oxide elements have a satisfactory voltage applied life characteristic. However, actual voltage applied ratio often exceeds the set value. Therefore, it is necessary to limit voltage distribution to 1.1 or below and voltage applied ratio to 90% or below. Thus the prior art is unable to provide a tank-type surge arrester employing high-resistance zinc oxide elements and capable of limiting voltage distribution to 1.1 or below.